1. Field of the Invention
The present invention relates to a guide element of a continuous casting plant, particularly a continuous casting plant for thin slabs, for guiding a metal strip in a strand guiding direction. The guide element includes a pair of guide rollers which are located opposite each other relative to the metal strip and form a roller gap.
2. Description of the Related Art
When continuously casting slabs, particularly at casting speeds of between 1.0 m/min and 10 m/min, it is of great importance for the casting safety as well as for the rolling process that the strand travels centrically through the strand guide section which is composed predominantly of rollers.
This centric travel of the strand during its solidification leads to
a safe casting pattern, i.e., strand ruptures in the mold area are avoided or suppressed, and
a symmetrical strand size which is important for producing a good hot strip profile with a crown of 1% and a good flatness.
This centric travel of the slab and the resulting high casting safety or symmetrical slab shape becomes important superproportionately with increasing casting speed and decreasing slab thickness.
Experience has shown that the control of a symmetrical crown or a symmetrical slab profile becomes more important as the thickness of the slab decreases or the width/thickness ratio of the slab increases because the width of the rolling stock does not further increase when the width/thickness ratio is about 100/1.
Accordingly, in the case of a thin slab having a solidification thickness of, for example, 150 to 40 mm, the control of the solidification profile becomes more important than for a standard slab of a thickness of 200 mm.
To be mentioned as prior art in this connection are the following patent applications:
PCT/DE 95/00 094 (WO 95/20 448)
PCT/DE 95/00 093 (WO 95/20 446)
PCT/DE 95/00 096 (WO 95/20 447)
In these applications, the roller strand guide sections have a concave shape.
In the two patent applications mentioned first, the rollers have a concave circumferential line, wherein the smallest diameter occurs in the middle of the roller.
In the third patent application PCT/DE 95/00 096, the concave roller strand guide section is formed by a bending of at most 8% which occurs under load.
This type of strand concavity cannot simultaneously meet the requirements of the strand in the area of solidification and after the solidification. The particular reasons for this are, for example, the reduction of the strand thickness during the solidification between the mold outlet and the final solidification, which in this technical field is known as continuous casting with liquid core, also called cast rolling or liquid core reduction. In this area of the solidification of the strand, the strand shell makes complete contact with the roller profile; this is caused by the prevailing ferrostatic pressure or the steel column present between the meniscus and the observed strand shell point.
After the last solidification point or after the final solidification, the strand is frozen in its shape, for example, with a profile whose thickness advantageously is 1% greater in the middle of the strand as compared to the narrow side wall areas, wherein the profile does not further change with respect to this percentage over the further path of the strand to the end of the strand guide section. The only change of the strand geometry occurs as a result of the shrinkage of the entire strand which is due to cooling.
The strand guide section must follow this shrinkage of the strand after the full solidification in order to ensure that the rollers have contact with the strand and, thus, the rollers continue to rotate.
Therefore, it is the primary object of the present invention to further develop a guide element of a continuous casting plant in such a way that the strand concavity can be adjusted in an optimum manner in the area of the solidification as well as after the solidification.
In accordance with the present invention, the pair of guide rollers is provided with an adjusting device for adjusting a crown of the roller gap.
As a result of this configuration, the roller gap can be adjusted to a defined crown which can be freely preselected as required.
By constructing the adjusting device to be capable of being operated in a pressure-controlled as well as a position-controlled manner, it is possible to ensure in a particularly simple manner the shaping in the strand portion with liquid core, on the one hand, and the roller contact in the fully solidified metal strip, on the other hand. In particular, the so-called liquid core reduction can be easily effected.
If at least one of the guide rollers has a center bearing, for example, because the guide roller is constructed as a divided guide roller, the adjusting device preferably is to be configured to act on the center bearing.
If, on the other hand, at least one of the guide rollers is constructed as a continuous guide roller, the adjusting device should act through at least one support roller on the at least one guide roller.
The support roller acts on the guide roller in a particularly reliable manner if the support roller is turned relative to the guide roller by a support angle which is different from zero.
In another embodiment, the adjusting device can also act through an additional support roller on the at least one guide roller, wherein the support roller and the additional support roller are arranged above and below the guide roller as seen in the strand guiding direction.
In accordance with another feature, the guide rollers and the adjusting device are mounted in frames which are capable of absorbing tension and pressure and the frames are adjustably tensioned relative to each other. This results in a particularly precise adjustability of the crown and of the total roll gap.
In accordance with another feature, at least one of the frames has a crossbeam for connecting the frame to a basic segment guide member through a fastening device which is arranged centrically relative to the crossbeam. As a result, the frame can expand symmetrically and unimpededly, for example, when heated.
The adjustment of the roller gap between the guide rollers is preferably effected by means of a hydraulic cushion. As is the case with the adjusting device, the hydraulic cushion also is preferably adjustable in a pressure-controlled and position-controlled manner.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.